1. Field of the Invention
The present invention relates generally to daylight cameras, and more particularly to vacuum feed assemblies which automatically transport photosensitive materials through the sequence of operations of such cameras.
2. Description of the Prior Art
The commercial appeal of a daylight camera that could automatically remove film from a storage receptacle, cut same to length, expose same, and transport the exposed film to a developer station has long been recognized. However, such objective has not yet been realized, although progress has been made in this quest by numerous conventional graphic arts systems. As an example, U.S. Pat. No. 4,089,603 granted May 16, 1978 to Robert D. Jacobs, and now assigned to Itek Graphix Corp. of Waltham, Mass., discloses a high speed vacuum support assembly for platemakers. The apparatus includes a first cover plate assembly having a transparent cover plate, and a second backing plate assembly including a relatively flexible backing sheet. The cover plate assembly is driven, by a motor, into contact with the photosensitive sheet supported therebetween, so that an airtight chamber is defined therebetween. The airtight chamber is evacuated (by a vacuum source) so that the flexible backing plate will be sucked toward the cover plate assembly, and drive the photosensitive sheet into film contact with the transparent cover plate.
U.S. Pat. No. 4,408,872, granted Oct. 11, 1983 to Quentin D. Vaughan, and assigned to Visual Graphics Corporation of Tamarac, Fla. discloses a vertically oriented, daylight camera system of particular relevance. This patent discloses a system in which a camera supports a table along which a cassette carriage is manually propelled. The carriage serves to store, and feed out, lengths of photosensitive material to a camera exposure station. A source of vacuum acting between a sheet of flexible, opaque plastic material and a translucent glass platen for the camera at the exposure station causes the film to be flattened against the platen prior to film exposure. Thereafter, vacuum is broken by movement of the carriage, and the exposed film is urged into the nip of rollers which carry the film through a developing station supported by the table. The exposed film is advanced toward the developing station by a brush-like member which picks up the trailing edge of the film; the brush-like member is secured beneath carriage 26.
U.S. Pat. No. 4,479,710, granted Oct. 30, 1984 to Quentin D. Vaughan, discloses refinements in the daylight camera system disclosed in U.S. Pat. No. 4,408,872, also granted to Quentin D. Vaughan and assigned to Visual Graphics Corporation of Tamarac, Fla. For example, U.S. Pat. No. 4,479,710 discloses protuberances formed on the underside of the flexible, opaque plastic sheet that is advanced by the manually operated cassette carriage across the platen of the camera; the protuberances insure smooth passage of the photosensitive material as it slides across the platen beneath the plastic sheet. U.S. Pat. No. 4,479,710 also discloses an elongated vacuum bar that assists in removing air from beneath the plastic sheet, as well as elongated plastic strips that enable the plastic sheet to be more easily lifted from the platen after the vacuum draw-down is terminated.
While the two patents granted to Quentin D. Vaughan represent improvements over previously known graphic arts systems, the Vaughan cameras rely upon manual operation of a cassette carriage in order to advance a length of photosensitive material, cut same to length, and advance the exposed film to a processing station. The back and forth movement of the cassette carriage across the table of the camera requires the continued presence of an operator, and increases the cost of operating the camera. The need to move the carriage in finite steps, to distinct locations on the table of the camera, calls for careful operation of the camera system, and reduces, potentially, the operational speed of the system. Also, to maintain the light-tight characteristic of the daylight camera system, Vaughan relies upon an elongated, flexible opaque sheet of plastic that is unwound from a roller situated at the rear of the cassette carriage; the effectiveness of this seal may diminish after extended use and stray light may enter the camera system.
U.S. Pat. Nos. 4,619,526 and 4,640,610 granted on Oct. 28, 1986 and Feb. 3, 1987, respectively, to Hougaard and Rasmussen and assigned to Eskofot A/S, disclose an automatic vertically oriented daylight camera system. The Eskofot patents disclose a vertical camera in which a sheet of photosensitive material is fed out and cut from a supply roll. As the sheet is cut, vacuum is activated to suction cups on the belt to grip the photosensitive sheet. The conveyor band is advanced while holding the photosensitive sheet until the photosensitive sheet is disposed at the desired location on the exposure platen. The vacuum to the suction cups is then terminated and vacuum is applied to longitudinal grooves adjacent to the exposure platen communicating with transverse grooves in the conveyor band to evacuate air under the belt, thereby holding the belt and photosensitive sheet against the platen during exposure. However, the structure disclosed in the Eskofot patents appears to simply drag the photosensitive sheet across the platen, subjecting the emulsion to scratching and other damage. In addition, Eskofot's attempt to move the sheet after exposure, even if accompanied by the "blow-under" of the 526 patent, would not appear to obtain reliable, repeatable release of the exposed sheet adhered to the platen.
Co-pending U.S. Pat. application Ser. No. 07/096,815 (attorney docket no. 1224-97B) filed in the name of Juergen Lein, commonly assigned herewith and entitled "Automatically Operated Vacuum Feed and Hold Down Assembly for Camera System" discloses a daylight camera system that automatically withdraws a length of photosensitive material from a supply roll, pauses while the material is cut to length, centers the sheet on the camera platen for exposure, assists in holding down the material for exposure, and then lifts the exposed sheet and advances the sheet to the camera processing station. The automatic feed assembly includes a transport belt wider than the platen and extending between first and second rollers at either end of the platen. The transport belt has two apertures operatively communicating with an air manifold block mounted to a vacuum pick-up bar secured to the top of the transport belt. The vacuum pick-up bar is attached to a belt lifting apparatus for raising and lowering the manifold block, belt and photosensitive material relative to the platen. The transport belt is a laminate of plastisol and polyester fabric with the plastisol removed or omitted in a mesh-screen "window area" of the belt conforming in shape to the platen but slightly larger in size. As there disclosed, the mesh screen area extends over the openings through the belt.
In the foregoing disclosure, a length of photosensitive material is fed from a supply cassette to a first position, where the belt is lowered and vacuum is applied through the manifold block and associated apertures to the underside of the belt, causing the photosensitive material to adhere to the belt. The lift mechanism is activated to lift the belt and photosensitive material and the belt is advanced to a second position to measure out the desired length of material. The sheet is then cut by a rotary cutter. Thereafter, the belt and sheet are advanced to a third position to center the sheet over the platen, where the belt and sheet are lowered so that the cut sheet contacts the glass exposure platen. At the third position suction to the manifold and belt apertures is terminated and vacuum is applied to a hold-down channel around the perimeter of the platen in order to pull the cut sheet and belt snugly against the platen for exposure. After exposure, the suction applied to the perimeter channel is terminated and a momentary supply of positive airflow to the channel breaks the residual vacuum between the belt and the platen. Suction is then reapplied to the manifold and belt apparatus and the belt lift mechanism is actuated to lift the belt and exposed sheet for transport to a fourth position. At the fourth position vacuum to the manifold and belt apertures is terminated as the cut sheet is taken up by the rollers of the development apparatus. Although the apparatus disclosed and claimed in the foregoing application marks a distinct improvement over prior structures, difficulties experienced in practicing that structure detract from the reliability of the structure there disclosed. Most notably, it has been found that supplying positive airflow to the channel does not always obtain reliable release of the exposed sheet from the platen, making it difficult if not impossible to advance the exposed sheet for processing. The previously mentioned Eskofot patents would appear to suffer from the same disadvantage.